Patent GB-1537484-A (GB)
Title: Transistor amplifier with over-current prevention circuitry
IPC: H03F3/30
Filed: 1976-01-27
Abstract: 1537484 Transistor amplifiers RCA CORPORATION 27 Jan 1976 [24 Feb 1975] 7657/75 Heading H3T An amplifier with overload protection, comprises a transistor, e.g. 181, in series with a load, a current source 17 for supplying a current proportional to the PD across the load between given limits, and means 11, 12 for supplying a portion of the current, linearly related to swings of an input signal of one polarity (negative) to the base of transistor 181, the proportionality factor of source 17 being predetermined to avoid operation of the transistor outside safe limits. The embodiment of Fig. 1 is a push-pull AF amplifier integrated circuit operating in Class AB. The current produced by source 17 comprises a component proportional to the collector current of common-base transistor 177, which receives its input from the amplifier output terminal 21, and a constant component determined by a bias voltage V A  applied to a transistor 171. The effect of the latter component is to provide a small collector current component (Iu) to output transistor 181, which is independent of the PD across 18, for charging or discharging any coupling capacitor connected between terminal 21 and the load. As the potential at 21 swings more positive the collector current of 177 decreases linearly until at a certain potential (V U ) 177 is cut off. As the potential at 21 swings less positive the collector current of 177 increases, until at a certain potential (V T ) a transistor 46 is switched on and begins to provide an emitter-follower input to the base of 177, with the result that the collector current of 177 becomes substantially constant for lower values of the potential at 21, corresponding to a maximum collector current (I max ) in 181. Current source 19 acts similarly on output transistor 201, transistors 197 and 47 corresponding, respectively, to 177 and 46, a current mirror 199 reproducing the collector current of 197 at the input of 19. An input signal at terminal 35 is fed to transistors 11 and 13 in parallel, biasing for the respective emitter coupled transistors 12 and 14 being provided by a network 60, such that transistors 11 and 14 supply small quiescent currents to 18 and 20, respectively, corresponding to Class AB operation. Network 60 comprises two current amplifiers 64, 69 forming a positive feedback loop. The current gain of 64, which may be unity at low currents, decreases as the current increases owing to the PD across resistor 63; in like manner the current gain of 69 may be unity at low currents and increases as the current increases owing to the effect of resistor 68. The current through resistors 63 and 70 stabilizes at a value corresponding to a reciprocal relation between the gains of 64 and 69, the corresponding biasing PD being proportional to the absolute temperature, which is the same as for the transistors 11- 14 by thermal contact. A potential regulator network 32-34 prevents forward biasing of the collector-base junction of 11 if 30 is driven to saturation. The network 18, substantially similar to 20, comprises a large transistor 181, which may be several in parallel, connected as a current mirror amplifier with a relatively small transistor 183. The low level gain of this amplifier is given by the ratio of the transconductances of 181 and 183, but the high level gain is equal to the resistance ratio 185 by 184, and hence is independent of transistor parameters and temperature. In another embodiment, Fig. 3 (not shown), the biasing of 11-14 is somewhat differently arranged, and current source 53 of Fig. 1 is not required.